When uninstalling Firefox, you will need to delete your Profile folder.
It's a hidden file.
This file is not
removed when you uninstall Firefox. It will retain all of your previous settings, and re-apply them to your new install.
Set windows to show hidden files,How to see hidden files in Windows
then navigate to:
C:\ Documents and Settings \ "your user name" \ Application Data \ Mozilla \ Firefox \ Profiles
Delete this folder (or send it to the Recycle Bin as a backup), and then try reinstalling Firefox.